coreCIFchem #5
Howard Flack crystal at flack.chMon Aug 9 18:52:04 BST 2004
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After e-mail correspondence between John Rutherford, David and myself, in my TNT cif replace: > _tecton_symmetry # point group if geometry present otherwise topological by two items (the table of values also needs modifying) _tecton_geometric_class # _tecton_graph_automorphism_group # _tecton_geometric_class is the orientation-independent specification of the tecton point group according to its geometry. It's probably best to use a Schoenflies symbol as there is no choice of basis implicit in the geometry as given by interatomic distances, interatomic (dihedral) angles. I suppose that if the geometry is given as a set of coordinates, it might be justifiable to specify a point group with a H-M point group symbol for the tecton orientation corresponding to the atomic coordinates. _tecton_graph_automorphism_group encodes the symmetry of the graph as a group of permutations (of atoms). If I understand correctly there are no standard symbols for these automorphism groups although it seems that in a fair number of cases they are isomorphic to a point group in three dimensions. So often one could use a Schoenflies symbol. [Even that is equivocal - point groups Cs, Ci and C2 are isomorphic] The TNT graph should be given the symbol C2v for its _tecton_graph_automorphism_group. [C2v is isomorphic to D2 and C2h. Why is it I prefer C2v? Am I (are we) too geometry oriented? I think we (David?) need to call John Rutherford again.] What one does in the case that the graph automorphism group is not isomorphic to a 3D point group, I do not have the least idea apart from writing a ? . H.
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